This invention relates to variable resistance cards, and more particularly to hybrid resistance cards (R-Cards) which are more versatile and adaptable, yet less expensive, than existing R-Cards, and methods for manufacturing same using a two-step printing process involving conductive and resistive inks.
R-Cards are a tapered resistive sheet, useful in a number of diverse applications. As shown in the prior art FIGS. 1 and 2, in a state of the art R-Card 10, the electrical resistivity is tapered across the width of the sheet 12, ranging from a relatively high (approximately air) resistance to a relatively low (approximately metal) resistance, following a desired resistivity curve (see FIG. 2). This resistive taper is repeated along the entire length of the sheet 12. Currently, a number of manufacturing processes are used to produce R-Cards. One of the most prevalent methods is to evaporate a conductive metal onto a kapton substrate. Another prevalent method involves sputtering onto a scrim material. Yet another method involves spraying material onto a substrate. However, these processes are not consistent from production lot to lot. Additionally, only linear lengths can be manufactured using the evaporated or sputtering method, since it is only possible to make an end-to-end taper, rather than being able to taper the resistance in any direction. Consequently, corners cannot be made in a continuous piece. As a result, R-Cards currently are mitered to cover curved surfaces, which is expensive, time consuming, and can be a source of performance degradation, since entire parts cannot be covered with one card.
Additional problems plaguing current state of the art R-Cards are numerous. Among them is that the resistivity curves that R-Cards currently follow are empirically manufactured at best. Current methods do not allow for flexibility in choosing the best substrate for the application. Furthermore, current production methods are very limited with regard to the high and low end resistances.
What is needed, therefore, are new manufacturing processes which will produce R-Cards that can smoothly follow a curvature without mitering, enabling coverage of an entire part with a single R-Card. Also needed are methods for producing R-Cards which can follow any resistivity curve accurately and which can be designed to start and stop at particular resistances. Most importantly, what is needed is a relatively low cost production process which produces consistently high quality R-Cards in every production run.